Apparatus for forming a positive electrostatic image

ABSTRACT

A machine for making a copy on a medium such as paper of a visual image. Particles of toner material are accelerated toward the paper through an apertured screen. Formed on the screen is a pattern of electrostatic charge regions that corresponds to the image so that the arrangement of the toner particles impinging on the paper corresponds to the image. A composite apertured screen composed of two conductive layers separated by an insulative layer and having an insulative layer on one outer surface. Means for establishing oppositely polarized fields across the inner insulative layer and across the outer insulative layer. One of the fields is selectively discharged to control the passage of toner particles through the apertures in the screen.

United States Patent 1191 Crane et al.

1 1 Jan. 30, 1973 [54] APPARATUS FOR FORMING A POSITIVE ELECTROSTATICIMAGE [75] Inventors: Hewitt D. Crane, Portola; Gerald L. Pressman,Cupertino; George J. Eilers, Redwood City, all of Calif.

FOREIGN PATENTS OR APPLICATIONS 553,355 2/1958 Canada ..355/3 1,156,30810/1963 Germany ..96/1 832 4/1966 Japan ..355/3 Primary ExaminerRobertP. Greiner Attorney-Stephen S. Townsend et al.

[57] ABSTRACT A machine for making a copy on a medium such as paper of avisual image. Particles of toner material are [52] U.S.Cl...355/3,355/4;)2/6l/l2, accelerated toward the p p through an apertured[51] Int. Cl. ..G03g 15/00 scren'hpormed 9 Screen is a g f deem) [58]Field of Search ..355/3,4, 16, 17; 96/1, 1.2 c We COmSP S f ""j that thearrangement of the toner part1cles 1mp1ng1ng 56 References Cited on thepaper corresponds to the image. A composite apertured screen composed oftwo conductwe layers UNITED STATES PATENTS separated by an insulativelayer and having an insula- 3,339,469 9/1967 McFarlane ..355/l6 fivelayer on Outer eeanstor establishing 3,532,422 10/1970 McFarlane..96/1.2 PP y Polanzed fields across the Inner msulatlve 3,582,2066/1971 Burdige ..355/16 layer and across the outer insulative layer. Oneof the 3,603,790 9/1971 Cleare ..355/17 fi ld i l tively discharged tocontrol the passage of toner particles through the apertures in thescreen.

4 Claims, 7 Drawing Figures 32 PAPER 20 CONDUCTIVE PLATE F 1 34 1 CORONASOURCE SHUTTER 3O IMAGE SOURCE k 24 SCREEN 1 TONER 14 l L CONTROL 55PATENTED JAN 30 mm 3.713; 7 34 SHEET 10F 3 32 PAPER 20 CONDUCTIVE PLATE*5 CORONA SHUTTER 33/ fiMRCE 53 26/ 30 IMAGE SOURCE CONTROL -53 52CONDUCTOR H 5/ A48 42 CONDUCTOR 40 )3 +6- NSULATOR 2::

( CONDUCTOR 5o 36 3a Icaeaseeecaeee INVENTORS HEWITT o. CRANE F IG 2 BYGERALD L'. PRESSMAN GEORGE J. EILERS Twmw 7 ATTORNEYS PAIENIEDJANZiOIQYS3,713,734

SHEET 2 0F 3 44 28 I6 PHOTO CONDUCTOR CONDUCTOR INSULATOR CONDUCTOR as36 so so 24 FIG 3 l I & 5.4615 f '6 ILLUMINATED AREA 44 40 l FIG 5INVENTORS HEWITT D. CRANE GERALD L. PRESSMAN BY GEORGE J. EILERS WWWATTORNEYS PATENTEDJAHBO 1m SHEET 3 0F 3 GLASS 7 T SPARENT C DUCTOR PH 0COND OR INVENTORS HEWITT D. CRANE GERALD L. PRESSMAN BY GEORGE J. EILERSATTORNEYS are exemplified in U.S. Pat. applications Ser. No.

673,499, filed Oct. 6, 1967, Ser. No. 776,146, filed Nov. 15, 1968, andSer. No. 85,070, filed Oct. 29, 1970, which are incorporated hereinto bythis reference. Such systems can include a support for a medium such aspaper on which it is desired to form a visible image. The system alsocan include a source of particles that have a color contrasting withthat of the medium. (Such particles are typically referred to as toner,and such usage will be employed hereinafter). In one type of system,there is interposed between the toner supply and the medium an aperturedscreen. An electrostatic field between the source of the toner particlesand the medium is established to propel the particles toward the mediumthrough the apertures in the screen. A plurality of electrostatic chargeregions is formed by the screen in a pattern that corresponds to theimage so that electrostatic fields within the apertures selectivelycontrol passage of toner particles. Toner particles egressing from theapertured screen therefore conform to the pattern of the image. Afterthe toner particles impinge on the medium they can be fixed or fusedthereon, if required, in accordance with known technology to provide apermanent image.

In another type of system, e.g., the systems described in U.S. Pat.applications Ser. No. 709,578, filed Mar. 1, I968; Ser. No. 864,022,filed Oct. 6, 1969; and Ser. No. 101,681, filed Dec. 28, I970, ions orlike charged particles are formed into a suitable pattern which patterncan be directed through a cloud of toner particles. The screen accordingto the present invention is useful in both types of system or in anyother system in which formation of a pattern of charged particles isrequired.

In the systems described above, the present invention provides animproved screen that offer certain advantages over various priorscreens. The screen construction according to the present inventionoperates at a relatively low voltage thereby simplifying fabrication andoperation of the device.

The embodiment of the invention described in more detail hereinafterincludes a composite apertured screen formed by four layers. On theupstream or obverse face of the screen, the side facing the source oftoner particles, the composite screen has a conductive layer underlyingwhich is an insulative layer. On the downstream face of the insulativelayer is a second conductive layer underlying which is an outerinsulative layer; the exposed surface of the outer insulative layerconstitutes the downstream or reverse face of the screen.

The improved screen is incorporated into the system by so biasing thetwo conductive layers that within the apertures is established anelectrostatic blocking field that has a polarity and magnitudesufficient to block the passage of toner particles through theapertures. The outer insulative layer is selectively electrostaticallycharged to a polarity and magnitude sufficient to form a field thatcounteracts and overrides portions of the blocking field to afi'ordpassage of toner particles in a pattern corresponding to an image.

In one satisfactory embodiment of the invention, the outer insulativelayer is formed of photoconductive material, a material that has a highresistance when in a dark state and a low resistance when in anilluminated state. While in the dark state, the photoconductive layer ischarged to a polarity and magnitude sufficient to create fields withinthe apertures that override or counteract the abovementioned blockingfield. Consequently, when the screen is in a dark state, all aperturesare biased to permit passage of toner particles traveling from the tonerparticle source to the medium. The image to be copied on the medium isthen projected on the photoconductive layer so that portions of thatlayer that correspond to light parts of the image assume a conductivestate. In such state, the charge previously formed on thephotoconductive layer is discharged thereby correspondingly dischargingthe counteracting field to permit the blocking field to becomeeffective. Consequently, apertures residing in such illuminated portionsblock the flow of toner particles whereas apertures associated with darkportions of the screen continue to permit passage of toner particles.With the screen in such condition a supply of charged toner particles isdirected through the screen to the medium as a consequence of which theparticles egressing from the screen are arranged in a pattern directlyor positively corresponding to the image.

The objects, features and advantages of the present invention will bemore apparent after referring to the following specification andaccompanying drawings in which:

FIG. I is a diagrammatic view of a system employing the screen of thepresent invention;

FIG. 2 is an enlarged fragmentary view of a screen according to thepresent invention showing the position of the electrostatic fieldswithin an aperture;

FIG. 3 is a fragmentary view of a larger portion of a screen constructedaccording to the present invention;

FIG. 4 is a fragmentary view similar to FIG. 3 showing the chargedcondition of the screen prior to receiving an image thereon;

FIG. 5 is a view similar to FIGS. 3 and 4 showing the charged conditionof the screen after a visible image has been impressed on thephotoconductive layer thereon;

FIG. 6 is a diagrammatic view similar to FIG. I showing an alternateembodiment of the invention; and

FIG. 7 is a diagrammatic view of another alternate embodiment of theinvention.

Referring more particularly to. the drawings and specifically to FIG. 1,reference numeral 12 schematically indicates an air and light imperviousenclosure in which a system employing the improved screen of theinvention is enclosed. Mounted within enclosure 12 is a source of tonerparticles 14, a composite apertured screen 16, an image receiving medium18 and a conductive support 20 for the medium. A power source 22 isprovided for establishing a field between toner source 14 and conductiveplate 20 so that toner particles are accelerated from .the toner sourceto the conductive plate. Screen 16 defines a large plurality ofapertures 24 in which apertures appropriate fields are establishedeither to block or to pass toner particles in accordance with the shapeof an image I to be formed on medium 18.

Within enclosure 12 is a charged particle source, such as a coronasource 26, which is employed to form a uniform charge on the reversesurface 28 of screen 24. After such uniform charge is formed, an imagesource 30 is projected onto the reverse surface of screen 24 through anoptical system that includes a reflector 32 and a shutter mechanism 34.Illumination of reverse face 28 of screen 24 alters the charge patternwithin the apertures of screen 24 so that toner particles egress fromthe screen apertures in a pattern corresponding to image source 30,whereby the image I is formed on medium 18.

A satisfactory embodiment of screen 16 is fragmentarily shown at greatlyenlarged size in FIG. 2. The screen includes an obverse face 36 thatfaces toward the source of toner particles. Obverse face 36 is definedby a conductive layer 38 underlying which is an insulative layer 40. Onthe surface of insulative layer 40 opposite from conductive layer 36 isa second or inner conductive layer 42; screen 16 is structurallycompleted by a layer 44 of photoconductive material the outer surface ofwhich defines reverse surface 28 of screen 16. In one screen designedaccording to the present invention insulative layer 40 andphotoconductive layer 44 have thicknesses of approximately 0.001- inchand conductive layers 38 and 42 are thin films deposited on suchinsulative layers. In such exemplary structure apertures 24 have adiameter of approximate ly 0.006-inch and as shown in the figure, theapertures extend through all layers so that each layer terminates in thewall bounding the apertures.

Blocking fields are formed within apertures 24 by establishing a bias orpotential between conductive layers 38 and 42 that has a magnitude andpolarity sufficient to form a charge across insulative 40 that createsthe blocking field. For this purpose, and assuming that toner particlesource 14 is biased to charge toner particles with a negative charge,conductive layer 38 is biased positively with respect to conductivelayer 42. Such bias is achieved by connecting a tap 46 in power source22 to conductive layer 38 and by connecting a tap 48 of the power sourceto conductive layer 42. The effect of such bias is to form at thesurface of insulative layer 40 that abuts conductive layer 38 aplurality of positive charges and at the surface of the insulator thatabuts conductive layer 42 a plurality of negative charges; the fringe ofsuch charges within each aperture 24 form an electrostatic field withinthe aperture identified by field lines 50 which have a magnitude andpolarization sufficient to block passage of toner particles through theaperture. In the exemplary structure referred to above, a potentialbetween conductive layers 38 and 42 of about 200-300 volts is consideredadequate to form a field within aperture 24 that totally blocks passageof toner particles through the aperture.

In order to permit passage of toner particles through aperture 24,the'present invention provides for creation of an electrostatic fieldthat counteracts or overrides the field designated by field lines 50. InFIG. 2 the field lines of the counteracting field are identified byreference numeral 52. Such counteracting field is formed by impressingon surface 28 of photoconductive layer 44, while such layer is in thedark or nonconductive state, charges of a suitable polarity, positivecharges in the case exemplified in the drawings. Such charges can beformed by any suitable expedient, for example, by bombardment of surface28 with positive ions from a corona wand, radioactive source, or thelike. In any event, the charges impressed on reverse surface 28 are of amagnitude and polarity such that the counteracting field indicated byfield lines 52 is formed by the cooperation of the charges on surface 28and the charges existing on the surfaces of inner insulative layer 40.In the example depicted in the drawings, the charges formed on reverseface 28 are more positive than the charges arising from the biassupplies connected to conductive layers 38 and 42.

The directional arrows associated with field lines 50 and 52 in thedrawings merely depict that the fields are oppositely polarized.Obviously, the direction of the field or the force resulting therefromdepends upon the polarity of the charge on the toner particles and noton any convention adopted for the purpose of illustrating the invention.Similarly, the blocking and counteracting fields may be viewed in thesingular or in the plural for a given screen.

The field formed between the charges on reverse face 28 and the chargeson the surfaces of inner insulator 40 cause fringe fields withinaperture 24 that have a magnitude and direction sufficient to counteractblocking field 50 to the end that the toner particles can pass throughaperture 24, a condition that subsists so long as conductive layer 44 isretained in a dark state.

Projection of image I onto reverse surface 28 illuminates all portionsof the conductive layer except those areas corresponding to the locationof dark lines and/ or areas of the image. Those portions ofphotoconductive layer 44 that are illuminated become locally conductiveso that current flow through the layer is permitted. Such current flowdischarges counteracting field 52 in all apertures 24 that areassociated with light portions of image I so that only blocking field 50exists in such apertures. In apertures corresponding to dark areas ofthe image, however, no current flow occurs through photoconductive layer44 as a consequence of which counteracting fields 52 subsist in suchapertures. Thus, even after removal of the image from reverse face 28,an electrostatic image remains on screen 16 so that when an acceleratingor projection field is activated to move toner particles from source 14to conductive plate 20, only apertures corresponding to dark portions ofimage I will admit and pass toner particles to medium 18. All otherapertures, i.e., apertures corresponding to regions of screen 16 thathave been illuminated, will block passage of toner particles since insuch apertures only blocking field 50 is effective. Consequently, apositive image is formed on medium 18 by the toner particles and suchimage can be made permanent by subsequent fixing or fusing of the tonerparticles on medium 18.

A typical image projected on. the reverse face of screen 16 includesregions that have no contrasting information, regions that arecompletely black (or other contrasting color), and regions that areintermediate the two extremes (e.g., varying degrees of gray). Thepresent invention permits accurate reproduction of all regions of theimage. When the image is projected on photoconductive layer 44, regionsof the layer become conductive in proportion to the intensity of lightimpinging thereon. Regions of the photoconductive layer corresponding tobright or highlight portions of the image are maximally illuminated andtherefore substantially totally discharged. Regions of thephotoconductive layer corresponding to gray portions of the image areonly partially illuminated and therefore only partially discharged.Regions of the photoconductive layer, corresponding to black portions ofthe image are not illuminated and therefore not discharged. Accordingly,passage of toner particles through various apertures 24 in screen 16occurs in direct proportion to the position and relative intensity inthe image.

The control functions referred to above are carried out in accordancewith conventional techniques and circuitry, and such conventionalcircuitry is indicated schematically in FIG. 1 by reference numeral 53.To recapitulate the operation of the present invention, reference is nowmade to FIGS. 3-5. FIG. 3 depicts screen 16 as it exists after powersource 22 has been activated. In such condition, a potential differencebetween inner electrode 42 and outer electrode 38 establishes blockingfields 50 within each aperture 24 so that any negatively charged tonerparticles approaching the obverse face, the lower face as viewed inFIGS. 3, 4 and 5, will be blocked and, therefore, will not pass throughthe apertures in the screen. FIG. 4 depicts the screen after a chargefrom particle source 26 has been impressed on photoconductive layer 44while the screen is in a dark condition. In such state, counteractingfields 52 are established within apertures 24 which fields fully orpartially override blocking fields 50 so that the net effect within eachaperture so affected is that toner particles approaching the screen willpass through the screen.

In FIG. 5, the region to the left labeled illuminated area and indicatedby reference numeral 54 corresponds with portions of. an image that areclear, or bright, whereas the remainder of the screen corresponds withportions of the image that are black or of some other contrasting color.In illuminated area 54, photoconductive layer 44 becomes locallyconductive and the charges thereon that sustain counteracting field 52are dissipated. Consequently, as to the apertures within the illuminatedarea, e.g., aperture 24a, blocking field 50 is solely effective so thattoner particles e.g., 14a, are repelled by the field and do not passthrough aperture 24a. As to the apertures that were not illuminated bythe image, however, the counteracting field remains effective tooverride the blocking field so that the toner particles e.g., 14b, canpass through such apertures. Those apertures in regions of the screenthat are partially illuminated will pass toner particles in reducedquantities. Accordingly, an image is formed downstream, i.e., above, asviewed in FIG. 5, of the plate 16 by toner particles identified byreference number 140.

Several alternate techniques for establishing counteracting fields areshown in FIGS. 6 and 7. In such figures, a modified screen 16 is shown.In such modified screen, conductive layers 38 and 42 and innerinsulative layer 40 are identical to the structure described above andtherefore bear identical reference numerals; in the modified screen,however, photoconductive layer 44 is replaced by an insulative ordielectric layer that is insensitive to light. Because layer 60 isformed of insulative material, it can store a charge in the dark or inthe light in the same manner that photoconductive layer 44 stores acharge when in a dark condition.

In the embodiment shown in FIG. 6, it is assumed that counteractingfields 52 have been established by impression of a charge on conductivelayer 60 in a manner equivalent to that described hereinabove; i.e., bybombardment from a corona source or the like. A multilayered dischargeplate 61 on which has been formed an electrostatic image correspondingto the visible image desired, is moved into contact with insulativelayer 60 for selectively discharging counteracting fields 52.

Plate 61 includes a transparent support layer 62 on one surface in whichis placed a thin, transparent conductive layer 64. Overlying transparentconductive layer 64 is a photoconductive layer 66'adapted for contactwith insulative layer 60 of screen 16. A suitable bias potential isapplied to the plate by connection to conductive layer 64. In a darkcondition, plate 61 is moved into contact with insulative layer 60 afterwhich the visible image is projected through transparent layers 62 and64 onto photoconductive layer 66. The regions of the photoconductivelayer that are illuminated become conductive and therefore dischargefields 52 on corresponding regions of plate 16'. Consequently, thecharge distribution on insulative layer 60 is selectively modified inaccordance with the image projected onto plate 61 so that upon removalofthe plate and acceleration of toner particles through screen 16', thetoner particles will be arranged in accordance with the visible image.

FIG. 7 shows still another modification wherein screen 16 is identicalto that described above in connection with FIG. 6. The embodiment ofFIG. 7 includes a plate 68 on which has been formed a latentelectrostatic image in accordance with the procedures disclosed in U.S.Pat. applications Ser. No. 673,499 and Ser. No. 776,I46. The charges onplate 68 that define an electrostatic latent image thereon aretransferred to the surface of insulative layer 60 of screen 16' when theplate is brought into physical contact with the screen. Such chargesform counteracting fields 52 in only those apertures through which it isdesired to admit toner particles. Thus, by employing the modification ofFIG. 7, the intermediate step of impressing counteracting field 52throughout the entire area of the screen is eliminated.

Thus, it will be seen that the present invention provides an improvedscreen for forming an electrostatic image which is positive in referenceto a visual image, to the end that positive images on a suitable mediumcan be formed by a screen according to this invention. Moreover, thescreen is capable of operation at relatively low voltages and is ofrelatively uncomplex construction.

Although several embodiments of the invention have been shown anddescribed, it will be obvious that other adaptations and modificationscan be made without departing from the true spirit and scope of theinvention.

What is claimed is:

1. Apparatus for forming an electrostatic image that corresponds to anoptical image comprising a composite screen defining a plurality ofspaced apart apertures and having an outer conductive layer forming theobverse face of said screen, an insulative layer superposed on saidouter conductive layer, an inner conductive layer superposed on saidinsulative layer, and a photoconductive layer superposed on said innerconductive layer and forming the reverse face of said screen, meansconnected between said outer conductive layer and said inner conductivelayer for establishing a voltage gradient across said insulative layer,said voltage gradient forming a first electrostatic field that extendsinto said apertures to form blocking fields within said aperturespolarized in a first direction, means for establishing a charge on saidreverse face that forms in cooperation with said inner and outerconductive layers a second field in said apertures that has a polarityand magnitude sufficient to counteract said first field when saidphotoconductive layer is in a dark condition, and means for impressingthe optical image on said reverse face so that relatively light regionsof said image cause corresponding regions of said photoconductive layerto become conductive and discharge corresponding portions of said secondfield.

2. Apparatus for forming a positive image of an object on a surfacecomprising a plurality of particles, means for forming an electric fieldbetween said particles and a particle receiving surface for acceleratingthe particles on a path toward said surface, a particle pervious screendisposed in said path in substantial parallelism with said surface, saidscreen having a reverse face facing said surface, an obverse faceopposite said reverse face, and a plurality of apertures extendingtherethrough between said obverse face and said reverse face, saidapertures being of sufficient size to admit said particles therethrough,said screen including an electrically conductive outer layer on saidobverse face, an electrically insulative layer underlying said outerconductive layer, an electrically conductive inner layer underlying saidinsulative layer and a photoconductive layer underlying said innerconductive layer and defining said reverse face, means for establishinga first electric field between said inner and outer conductive layer sopolarized as to repel particles approaching the apertures from thedirection of said obverse face, means for establishing a second electricfield between said inner conductive layer and said photoconductive layerthat has a polarity and magnitude sufficient to counteract said firstfield, and means for projecting an image of the object on said reverseface so that portions of said photoconductive layer that are illuminatedwill establish a conductive path to discharge said second field in theilluminated portions, whereby particles are repelled by said first fieldat apertures in such illuminated portions of the screen.

3. A system for controlling the flow of a stream of charged particlescomprising:

means for establishing a coplanar array of a first plurality of bipolarelectrostatic fields uniformly oriented and having a strength sufficientto prevent a flow of a stream of charged particles directed throu h thearray; means for estab ishmg a coplanar array of a second plurality ofbipolar electrostatic fields, said second plurality of fields beinguniformly oriented in a direction opposite said first plurality offields, said second plurality of electrostatic fields being superimposedover said first plurality of fields and having a strength sufficient tocounteract said first plurality of fields;

and means for selectively dissipating said second plurality of bipolarfields thereby to modulate the fiow of a stream of charged particlesdirected through said arrays in accordance with an image to bereproduced.

4. Apparatus for forming a positive image of an object on a surface witha plurality of particles comprising means for forming an electric fieldbetween the particles and a particle receiving surface for acceleratingthe particles on a path toward said surface, a particle pervious screendisposed in said path in substantial parallelism with said surface, saidscreen having a reverse face facing said surface, an obverse faceopposite said reverse face, and a' plurality of apertures extendingtherethrough between said obverse face and said reverse face, saidapertures being of sufficient size to admit said particles therethrough,said screen including an electrically conductive outer layer on saidobverse face, an electrically insulative layer underlying said outerconductive layer, an electrically conductive inner layer underlying saidinsulative layer and a photoconductive layer underlying said innerconductive layer and defining said reverse face, means for establishinga first electric field between said inner and outer conductive layer sopolarized as to repel particles approaching the apertures from thedirection of said obverse face, means for establishing a second electricfield between said inner conductive layer and said photoconductive layerthat has a polarity and magnitude sufficient to counteract said firstfield, and means for projecting an image of the object on said reverseface so that portions of said photoconductive layer that are illuminatedwill establish a conductive path to discharge said second field in theilluminated portions, whereby particles are repelled by said first fieldat apertures in such illuminated portions of the screen.

1. Apparatus for forming an electrostatic image that corresponds to anoptical image comprising a composite screen defining a plurality ofspaced apart apertures and having an outer conductive layer forming theobverse face of said screen, an insulative layer superposed on saidouter conductive layer, an inner conductive layer superposed on saidinsulative layer, and a photoconductive layer superposed on said innerconductive layer and forming the reverse face of said screen, meansconnected between said outer conductive layer and said inner conductivelayer for establishing a voltage gradient across said insulative layer,said voltage gradient forming a first electrostatic field that extendsinto said apertures to form blocking fields within said aperturespolarized in a first direction, means for establishing a charge on saidreverse face that forms in cooperation with said inner and outerconductive layers a second field in said apertures that has a polarityand magnitude sufficient to counteract said first field when saidphotoconductive layer is in a dark condition, and means for impressingthe optical image on said reverse face so that relatively light regionsof said image cause corresponding regions of said photoconductive layerto become conductive and discharge corresponding portions of said secondfield.
 2. Apparatus for forming a positive image of an object on asurface comprising a plurality of particles, means for forming anelectric field between said particles and a particle receiving surfacefor accelerating the particles on a path toward said surface, a particlepervious screen disposed in said path in substantial parallelism withsaid surface, said screen having a reverse face facing said surface, anobverse face opposite said reverse face, and a plurality of aperturesextending therethrough between said obverse face and said reverse face,said apertures being of sufficient size to admit said particlestherethrough, said screen including an electrically conductive outerlayer on said obverse face, an electrically insulative layer underlyingsaid outer conductive layer, aN electrically conductive inner layerunderlying said insulative layer and a photoconductive layer underlyingsaid inner conductive layer and defining said reverse face, means forestablishing a first electric field between said inner and outerconductive layer so polarized as to repel particles approaching theapertures from the direction of said obverse face, means forestablishing a second electric field between said inner conductive layerand said photoconductive layer that has a polarity and magnitudesufficient to counteract said first field, and means for projecting animage of the object on said reverse face so that portions of saidphotoconductive layer that are illuminated will establish a conductivepath to discharge said second field in the illuminated portions, wherebyparticles are repelled by said first field at apertures in suchilluminated portions of the screen.
 3. A system for controlling the flowof a stream of charged particles comprising: means for establishing acoplanar array of a first plurality of bipolar electrostatic fieldsuniformly oriented and having a strength sufficient to prevent a flow ofa stream of charged particles directed through the array; means forestablishing a coplanar array of a second plurality of bipolarelectrostatic fields, said second plurality of fields being uniformlyoriented in a direction opposite said first plurality of fields, saidsecond plurality of electrostatic fields being superimposed over saidfirst plurality of fields and having a strength sufficient to counteractsaid first plurality of fields; and means for selectively dissipatingsaid second plurality of bipolar fields thereby to modulate the flow ofa stream of charged particles directed through said arrays in accordancewith an image to be reproduced.